Para-selective zeolite catalysts treated with carbon dioxide

ABSTRACT

A process is provided for modifying ZSM-5 type zeolite catalysts with carbon dioxide treating agent in order to enhance the para-selective properties of such catalysts for the conversion of aromatic materials to dialkyl-substituted benzene compounds. Catalyst compositions so treated can be used in alkylation, transalkylation or disproportionation processes to provide product mixtures having exceptionally high concentrations of the para-dialkylbenzene isomer.

This is a division of application Ser. No. 277,484 filed June 26, 1981,now U.S. Pat. No. 4,367,359.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the preparation and use of modifiedzeolite catalyst compositions which are especially suitable for theconversion of substituted aromatic hydrocarbons to provide productmixtures enriched in the para-(or 1,4-) dialkyl substituted benzeneisomer.

2. Description of the Prior Art

Production of dialkyl substituted benzene compounds viadisproportionation, alkylation and/or transalkylation of aromatichydrocarbons is an important step in a number of commercial chemicalmanufacturing processes. Such reactions can be carried out over avariety of catalyst materials. Alkylation of aromatic hydrocarbonsutilizing crystalline aluminosilicate catalysts has, for example, beendescribed. U.S. Pat. No. 2,904,607 to Mattox refers to alkylation ofaromatic hydrocarbons with an olefin in the presence of a crystallinemetallic aluminosilicate having uniform openings of about 6 to 15Angstrom units. U.S. Pat. No. 3,251,897 to Wise describes alkylation ofaromatic hydrocarbons in the presence of X or Y-type crystallinealuminosilicate zeolites, specifically such type zeolites wherein thecation is rare earth and/or hydrogen. U.S. Pat. No. 4,086,287 to Kaedinget al discloses the use of ZSM-5 type zeolites as catalysts for thealkylation of aromatic hydrocarbons such as toluene. U.S. Pat. No.3,751,504 to Keown et al. and U.S. Pat. No. 3,751,506 to Burressdescribe vapor phase alkylation of aromatic hydrocarbons with olefins,e.g., benzene with ethylene, in the presence of a ZSM-5 type zeolitecatalyst.

The disproportionation of aromatic hydrocarbons in the presence ofzeolite catalysts has been described by Grandio et al in the Oil and GasJournal, Vol. 69, No. 48 (1971), U.S. Pat. Nos. 3,126,422; 3,413,374,3,598,878; 3,598,879 and 3,607,961 show vapor-phase disproportionationof toluene over various catalysts.

In many of these prior art processes, the dialkylbenzene productproduced contains more of the 1,3-isomer than either of the other twoisomers. In the conventional methylation of toluene to form xylene, theproduct has the equilibrium composition of approximately 24 percent of1,4-, 54 percent of 1,3- and 22 percent of 1,2-isomer. Of thedialkylbenzene isomers, 1,3-dialkylbenzene is often the least desiredproduct, with 1,2- and 1,4-dialkylbenzene being the more usefulproducts. 1,4-Dimethylbenzene, for example, is of particular value,being useful in the manufacture of terephthalic acid which is anintermediate in the manufacture of synthetic fibers such as "Dacron".Furthermore, 1,4- methylethylbenzene, i.e., para-ethyltoluene (PET), isuseful for subsequent conversion to para-methylstyrene, and for thispurpose ethyltoluene products containing as much as 97% of the paraisomer are required.

Mixtures of dialkylbenzene isomers, either alone or in further admixturewith ethylbenzene, have previously been separated by expensivesuperfractionation and multistage refrigeration steps. Such processes,as will be realized, involve high operation costs and have a limitedyield. Alternatively, various modified zeolite catalysts have beendeveloped to alkylate or disproportionate toluene with a greater orlesser degree of selectivity to 1,4-dialkylbenzene isomers. Hence, U.S.Pat. Nos. 3,972,832, 4,034,053, 4,128,592, and 4,137,195 discloseparticular zeolite catalysts which have been treated with compounds ofphosphorus and/or magnesium to increase para-selectivity of thecatalysts. Para selective boron-containing zeolites are shown in U.S.Pat. No. 4,067,920 and para-selective, antimony-containing zeolites inU.S. Pat. No. 3,979,472. Similarly, U.S. Pat. Nos. 3,965,208 and4,117,026 disclose other modified zeolites useful for shape selectivereactions.

Notwithstanding the existence of such modified zeolite catalysts havingpara-selective properties, there is a continuing need to developadditional types of catalytic materials which are highly para-selectivewhen used for the conversion of aromatic compounds to dialkylbenzeneproducts. Accordingly, it is an object of the present invention toprovide modified zeolite catalyst compositions which promote theconversion of aromatics to produce mixtures containing an exceptionallyhigh percentage, e.g., 97% by weight or more, for alkylation of toluene,of para-dialkylbenzene isomer.

It is a further object of the present invention to provide such highlypara-selective catalysts without necessarily resorting to expensiveand/or time consuming catalyst selectivation techniques such as steamingand/or precoking after each instance of catalyst regeneration.

It is a further object of the present invention to provide highlypara-selective alkylation, transalkylation and disproportionationprocesses employing the modified zeolite catalysts described herein.

SUMMARY OF THE INVENTION

The present invention relates to a process for modifying zeolitecatalysts to render such catalysts highly para-selective for theconversion of aromatic compounds dialkyl substituted benzene compounds.The zeolite component of the catalysts so modified is one having asilica to alumina mole ratio of at least 12 and a constraint indexwithin the approximate range of 1 to 12. Such zeolite catalysts aremodified by treatment with carbon dioxide under catalyst treatingconditions which enhance catalyst para-selectivity.

The present invention also relates to catalyst compositions modified inthis manner and to alkylation, transalkylation and disproportionationprocesses utilizing such modified catalyst compositions.

DETAILED DESCRIPTION OF THE INVENTION

The crystalline zeolites used in the present invention are members of aparticular class of zeolitic materials which exhibit unusual properties.Although these zeolites have unusually low alumina contents, i.e. highsilica to alumina mole ratios, they are very active even when the silicato alumina mole ratio exceeds 30. The activity is surprising, sincecatalytic activity is generally attributed to framework aluminum atomsand/or cations associated with these aluminum atoms. These zeolitesretain their crystallinity for long periods in spite of the presence ofsteam at high temperature which induces irreversible collapse of theframework of other zeolites, e.g. of the X and A type. Furthermore,carbonaceous deposits, when formed, may be removed by burning at higherthan usual temperatures to restore activity. These zeolites, used ascatalysts, generally have low coke-forming activity and therefore areconducive to long times on stream between regenerations by burningcarbonaceous deposits with oxygen-containing gas such as air.

An important characteristic of the crystal structure of this particularclass of zeolites is that it provides a selective constrained access toand egress from the intracrystalline free space by virtue of having aneffective pore size intermediate between the small pore Linde A and thelarge pore Linde X, i.e. the pore windows of the structure are of abouta size such as would be provided by 10-membered rings of silicon atomsinterconnected by oxygen atoms. It is to be understood, of course, thatthese rings are those formed by the regular disposition of thetetrahedra making up the anionic framework of the crystalline zeolite,the oxygen atoms themselves being bonded to the silicon (or aluminum,etc.) atoms at the centers of the tetrahedra.

The silica to alumina mole ratio referred to may be determined byconventional analysis. This ratio is meant to represent, as closely aspossible, the ratio in the rigid anionic framework of the zeolitecrystal and to exclude aluminum in the binder or in cationic or otherform within the channels. Although zeolites with a silica to aluminamole ratio of at least 12 are useful, it is preferred in some instancesto use zeolites having substantially higher silica/alumina ratios, e.g.1600 and above. In addition, zeolites as otherwise characterized hereinbut which are substantially free of aluminum, that is zeolites havingsilica to alumina mole ratios of up to infinity, are found to be usefuland even preferable in some instances. Such "high silica" or "highlysiliceous" zeolites are intended to be included within this description.Also to be included within this definition are substantially pure silicaanalogs of the useful zeolites described herein, that is to say thosezeolites having no measurable amount of aluminum (silica to alumina moleratio of infinity) but which otherwise embody the characteristicsdisclosed.

Members of this particular class of zeolites, after activation, acquirean intracrystalline sorption capacity for normal hexane which is greaterthan that for water, i.e. they exhibit "hydrophobic" properties. Thishydrophobic character can be used to advantage in some applications.

Zeolites of the particular class useful herein have an effective poresize such as to freely sorb normal hexane. In addition, their structuremust provide constrained access to larger molecules. It is sometimespossible to judge from a known crystal structure whether suchconstrained access exists. For example, if the only pore windows in acrystal are formed by 8-membered rings of silicon and aluminum atoms,then access by molecules of larger cross-section than normal hexane isexcluded and the zeolite is not of the desired type. Windows of10-membered rings are preferred, although in some instances excessivepuckering of the rings or pore blockage may render these zeolitesineffective.

Although 12-membered rings in theory would not offer sufficientconstraint to produce advantageous conversions, it is noted that thepuckered 12-ring structure of TMA offretite does show some constrainedaccess. Other 12-ring structures may exist which may be operative forother reasons and, therefore, it is not the present intention toentirely judge the usefulness of a particular zeolite solely fromtheoretical structural considerations.

Rather than attempt to judge from crystal structure whether or not azeolite possesses the necessary constrained access to molecules oflarger cross-section than normal paraffins, a simple determination ofthe "Constraint Index" as herein defined may be made by passingcontinuously a mixture of an equal weight of normal hexane and3-methylpentane over a sample of zeolite at atmospheric pressureaccording to the following procedure. A sample of the zeolite, in theform of pellets or extrudate, is crushed to a particle size about thatof coarse sand and mounted in a glass tube. Prior to testing, thezeolite is treated with a stream of air at 540° C. for at least 15minutes. The zeolite is then flushed with helium and the temperature isadjusted between 290° C. and 510° C. to give an overall conversion ofbetween 10% and 60%. The mixture of hydrocarbons is passed at 1 liquidhourly space velocity (i.e., 1 volume of liquid hydrocarbon per volumeof zeolite per hour) over the zeolite with a helium dilution to give ahelium to (total) hydrocarbon mole ratio of 4:1. After 20 minutes onstream, a sample of the effluent is taken and analyzed, mostconveniently by gas chromatography, to determine the fraction remainingunchanged for each of the two hydrocarbons.

While the above experimental procedure will enable one to achieve thedesired overall conversion of 10 to 60% for most zeolite samples andrepresents preferred conditions, it may occasionally be necessary to usesomewhat more severe conditions for samples of very low activity, suchas those having an exceptionally high silica to alumina mole ratio. Inthose instances, a temperature of up to about 540° C. and a liquidhourly space velocity of less than one, such as 0.1 or less, can beemployed in order to achieve a minimum total conversion of about 10%.

The "Constraint Index" is calculated as follows: ##EQU1##

The Constraint Index approximates the ratio of the cracking rateconstants for the two hydrocarbons. Zeolites suitable for the presentinvention are those having a Constraint Index of about 1 to 12.Constraint Index (CI) values for some typical materials are:

    ______________________________________                                                          C.I.                                                        ______________________________________                                        ZSM-4               0.5                                                       ZSM-5               8.3                                                       ZSM-11              8.7                                                       ZSM-12              2                                                         ZSM-23              9.1                                                       ZSM-35              4.5                                                       ZSM-38              2                                                         ZSM-48              3.4                                                       TMA Offretite       3.7                                                       Clinoptilolite      3.4                                                       H--Zeolon (mordenite)                                                                             0.4                                                       REY                 0.4                                                       Amorphous Silica-Alumina                                                                          0.6                                                       Erionite            38                                                        ______________________________________                                    

The above-described Constraint Index is an important and even criticaldefinition of those zeolites which are useful in the instant invention.The very nature of this parameter and the recited technique by which itis determined, however, admit of the possibility that a given zeolitecan be tested under somewhat different conditions and thereby exhibitdifferent Constraint Indices. Constraint Index seems to vary somewhatwith severity of operation (conversion) and the presence or absence ofbinders. Likewise, other variables such as crystal size of the zeolite,the presence of occluded contaminants, etc., may affect the constraintindex. Therefore, it will be appreciated that it may be possible to soselect test conditions as to establish more than one value in the rangeof 1 to 12 for the Constraint Index of a particular zeolite. Such azeolite exhibits the constrained access as herein defined and is to beregarded as having a Constraint Index in the range of 1 to 12. Alsocontemplated herein as having a Constraint Index in the range of 1 to 12and therefore within the scope of the defined class of highly siliceouszeolites are those zeolites which, when tested under two or more sets ofconditions within the above-specified ranges of temperature andconversion, produce a value of the Constraint Index slightly less than1, e.g. 0.9, or somewhat greater than 12, e.g. 14 or 15, with at leastone other value within the range of 1 to 12. Thus, it should beunderstood that the Constraint Index value as used herein is aninclusive rather than a exclusive value. That is, a crystalline zeolitewhen identified by any combination of conditions within the testingdefinition set forth herein as having a Constraint Index in the range of1 to 12 is intended to be included in the instant novel zeolitedefinition whether or not the same identical zeolite, when tested underother of the defined conditions, may give a Constraint Index valueoutside of the range of 1 to 12.

The particular class of zeolites defined herein is exemplified by ZSM-5,ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, ZSM-48 and other similarmaterials.

ZSM-5 is described in greater detail in U.S. Pat. Nos. 3,702,886 and Re29,948. The entire descriptions contained within those patents,particularly the X-ray diffraction pattern of therein disclosed ZSM-5,are incorporated herein by reference.

ZSM-11 is described in U.S. Pat. No. 3,709,979. That description, and inparticular the X-ray diffraction pattern of said ZSM-11, is incorporatedherein by reference.

ZSM-12 is described in U.S. Pat. No. 3,832,449. That description, and inparticular the X-ray diffraction pattern disclosed therein, isincorporated herein by reference.

ZSM-23 is described in U.S. Pat. No. 4,076,842. The entire contentthereof, particularly the specification of the X-ray diffraction patternof the disclosed zeolite, is incorporated herein by reference.

ZSM-35 is described in U.S. Pat. No. 4,016,245. The description of thatzeolite, and particularly the X-ray diffraction pattern thereof, isincorporated herein by reference.

ZSM-38 is more particularly described in U.S. Pat. No. 4,046,859. Thedescription of that zeolite, and particularly the specified X-raydiffraction pattern thereof, is incorporated herein by reference.

ZSM-48 can be identified, in terms of moles of anhydrous oxides per 100moles of silica, as follows:

    (0-15)RN:(0-1.5)M.sub.2/n O:(0-2)Al.sub.2 O.sub.3 :(100)SiO.sub.2

wherein:

M is at least one cation having a valence n; and

RN is a C₁ -C₂₀ organic compound having at least one amine functionalgroup of pK_(a) ≧7.

It is recognized that, particularly when the composition containstetrahedral, framework aluminum, a fraction of the amine functionalgroups may be protonated. The doubly protonated form, in conventionalnotation, would be (RNH)₂ O and is equivalent in stoichiometry to 2RN+H₂O.

The characteristic X-ray diffraction pattern of the synthetic zeoliteZSM-48 has the following significant lines:

    ______________________________________                                        Characteristic Lines of ZSM-48                                                d(A)       Relative Intensity                                                 ______________________________________                                        11.9       W-S                                                                10.2       W                                                                  7.2        W                                                                  5.9        W                                                                  4.2        VS                                                                 3.9        VS                                                                 3.6        W                                                                  2.85       W                                                                  ______________________________________                                    

These values were determined by standard techniques. The radiation wasthe K-alpha doublet of copper, and a scintillation counter spectrometerwith a strip chart pen recorder was used. The peak heights, I, and thepositions as a function of 2 times theta, where theta is the Braggangle, were read from the spectrometer chart. From these, the relativeintensities, 100 I/I_(o), where I_(o) is the intensity of the strongestline or peak, and d (obs.), the interplanar spacing in A, correspondingto the recorded lines, were calculated. In the foregoing table therelative intensities are given in terms of the symbols W=weak, VS=verystrong and W-S=weak-to-strong. Ion exchange of the sodium with cationsreveals substantially the same pattern with some minor shifts ininterplanar spacing and variation in relative intensity. Other minorvariations can occur depending on the silicon to aluminum ratio of theparticular sample, as well as if it has been subjected to thermaltreatment.

The ZSM-48 can be prepared from a reaction mixture containing a sourceof silica, water, RN, an alkali metal oxide (e.g. sodium) and optionallyalumina. The reaction mixture should have a composition, in terms ofmole ratios of oxides, falling within the following ranges:

    ______________________________________                                        REACTANTS       BROAD      PREFERRED                                          ______________________________________                                        Al.sub.2 O.sub.3 /SiO.sub.2 =                                                                  0 to 0.02 0 to 0.01                                          Na/SiO.sub.2 =  0 to 2     0.1 to 1.0                                         RN/SiO.sub.2 =  0.01 to 2.0                                                                              0.05 to 1.0                                        OH.sup.- /SiO.sub.2 =                                                                         0 to 0.25  0 to 0.1                                           H.sub.2 O/SiO.sub.2 =                                                                         10 to 100  20 to 70                                           H.sup.+ (added)/SiO.sub.2 =                                                                   0 to 0.2   0 to 0.05                                          ______________________________________                                    

wherein RN is a C₁ -C₂₀ organic compound having an amine functionalgroup of pK_(a) ≧7. The mixture is maintained at 80°-250° C. untilcrystals of the material are formed. H⁺ (added) is moles acid added inexcess of the moles of hydroxide added. In calculating H⁺ (added) and OHvalues, the term acid (H⁺) includes both hydronium ion, whether free orcoordinated, and aluminum. Thus aluminum sulfate, for example, would beconsidered a mixture of aluminum oxide, sulfuric acid, and water. Anamine hydrochloride would be a mixture of amine and HCl. In preparingthe highly siliceous form of ZSM-48 no alumina is added. Thus, the onlyaluminum present occurs as an impurity in the reactants.

Preferably, crystallization is carried out under pressure in anautoclave or static bomb reactor, at 80° C. to 250° C. Thereafter, thecrystals are separated from the liquid and recovered. The compositioncan be prepared utilizing materials which supply the appropriate oxide.Such compositions include sodium silicate, silica hydrosol, silica gel,silicic acid, RN, sodium hydroxide, sodium chloride, aluminum sulfate,sodium aluminate, aluminum oxide, or aluminum itself. RN is a C₁ -C₂₀organic compound containing at least one amine functional group ofpK_(a) ≧7, as defined above, and includes such compounds as C₃ -C₁₈primary, secondary, and tertiary amines, cyclic amine (such aspiperidine, pyrrolidine and piperazine), and polyamines such as NH₂-C_(n) H_(2n) -NH₂, wherein n is 4-12.

In all of the foregoing zeolites, the original cations can besubsequently replaced, at least in part, by calcination and/or ionexchange with another cation. Thus, the original cations can beexchanged into a hydrogen or hydrogen ion precursor form or a form inwhich the original cations have been replaced by a metal of, forexample, Groups II through VIII of the Periodic Table. Thus, it iscontemplated to exchange the original cations with ammonium ions or withhydronium ions. Catalytically active forms of these would include, inparticular, hydrogen, rare earth metals, aluminum, manganese and othermetals of Groups II and VIII of the Periodic Table.

It is to be understood that by incorporating by reference the foregoingpatents to describe examples of specific members of the specifiedzeolite class with greater particularity, it is intended thatidentification of the therein disclosed crystalline zeolites be resolvedon the basis of their respective X-ray diffraction patterns. Asdiscussed above, the present invention contemplates utilization of suchcatalysts wherein the mole ratio of silica to alumina is essentiallyunbounded. The incorporation of the identified patents should thereforenot be construed as limiting the disclosed crystalline zeolites to thosehaving the specific silica-alumina mole ratios discussed therein, it nowbeing known that such zeolites may be substantially aluminum-free andyet, having the same crystal structure as the disclosed materials, maybe useful or even preferred in some applications. It is the crystalstructure, as identified by the X-ray diffraction "fingerprint", whichestablishes the identity of the specific crystalline zeolite material.

The specific zeolites described, when prepared in the presence oforganic cations, are substantially catalytically inactive, possiblybecause the intra-crystalline free space is occupied by organic cationsfrom the forming solution. They may be activated by heating in an inertatmosphere at 540° C. for one hour, for example, followed by baseexchange with ammonium salts followed by calcination at 540° C. in air.The presence of organic cations in the forming solution may not beabsolutely essential to the formation of this type zeolite; however, thepresence of these cations does appear to favor the formation of thisspecial class of zeolite. More generally, it is desirable to activatethis type catalyst by base exchange with ammonium salts followed bycalcination in air at about 540° C. for from about 15 minutes to about24 hours.

Natural zeolites may sometimes be converted to zeolite structures of theclass herein identified by various activation procedures and othertreatments such as base exchange, steaming, alumina extraction andcalcination, alone or in combinations. Natural minerals which may be sotreated include ferrierite, brewsterite, stilbite, dachiardite,epistilbite, heulandite, and clinoptilolite.

The preferred crystalline zeolites for utilization herein include ZSM-5,ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 and ZSM-48, with ZSM-5 beingparticularly preferred.

In a preferred aspect of this invention, the zeolites hereof areselected as those providing among other things a crystal frameworkdensity, in the dry hydrogen form, of not less than about 1.6 grams percubic centimeter. It has been found that zeolites which satisfy allthree of the discussed criteria are most desired for several reasons.When hydrocarbon products or by-products are catalytically formed, forexample, such zeolites tend to maximize the production of gasolineboiling range hydrocarbon products. Therefore, the preferred zeolitesuseful with respect to this invention are those having a ConstraintIndex as defined above of about 1 to about 12, a silica to alumina moleratio of at least about 12 and a dried crystal density of not less thanabout 1.6 grams per cubic centimeter. The dry density for knownstructures may be calculated from the number of silicon plus aluminumatoms per 1000 cubic Angstroms, as given, e.g., on Page 19 of thearticle ZEOLITE STRUCTURE by W. M. Meier. This paper, the entirecontents of which are incorporated herein by reference, is included inPROCEEDINGS OF THE CONFERENCE ON MOLECULAR SIEVES, (London, April 1967)published by the Society of Chemical Industry, London, 1968.

When the crystal structure is unknown, the crystal framework density maybe determined by classical pyknometer techniques. For example, it may bedetermined by immersing the dry hydrogen form of the zeolite in anorganic solvent which is not sorbed by the crystal. Or, the crystaldensity may be determined by mercury porosimetry, since mercury willfill the interstices between crystals but will not penetrate theintracrystalline free space.

It is possible that the unusual sustained activity and stability of thisspecial class of zeolites is associated with its high crystal anionicframework density of not less than about 1.6 grams per cubic centimeter.This high density must necessarily be associated with a relatively smallamount of free space within the crystal, which might be expected toresult in more stable structures. This free space, however, is importantas the locus of catalytic activity.

Crystal framework densities of some typical zeolites, including somewhich are not within the purview of this invention, are:

    ______________________________________                                                    Void       Framework                                                          Volume     Density                                                ______________________________________                                        Ferrierite    0.28 cc/cc   1.76 g/cc                                          Mordenite     .28          1.7                                                ZSM-5, -11    .29          1.79                                               ZSM-12        --           1.8                                                ZSM-23        --           2.0                                                Dachiardite   .32          1.72                                               L             .32          1.61                                               Clinoptilolite                                                                              .34          1.71                                               Laumontite    .34          1.77                                               ZSM-4 (Omega) .38          1.65                                               Heulandite    .39          1.69                                               P             .41          1.57                                               Offretite     .40          1.55                                               Levynite      .40          1.54                                               Erionite      .35          1.51                                               Gmelinite     .44          1.46                                               Chabazite     .47          1.45                                               A             .5           1.3                                                Y             .48          1.27                                               ______________________________________                                    

When synthesized in the alkali metal form, the zeolite is convenientlyconverted to the hydrogen form, generally by intermediate formation ofthe ammonium form as a result of ammonium ion exchange and calcinationof the ammonium form to yield the hydrogen form. In addition to thehydrogen form, other forms of the zeolite wherein the original alkalimetal has been reduced to less than about 1.5 percent by weight may beused as precursors to the alkaline-earth metal modified zeolites of thepresent invention. Thus, the original alkali metal of the zeolite may bereplaced by ion exchange with other suitable metal cations of Groups Ithrough VIII of the Periodic Table, including, by way of example,nickel, copper, zinc, palladium, calcium or rare earth metals.

In practicing any given desired hydrocarbon conversion process includingthose of the present invention, it may be useful to incorporate theabove-described crystalline zeolites with a matrix comprising anothermaterial resistant to the temperature and other conditions employed inthe process. Such matrix material is useful as a binder and impartsgreater resistance to the catalyst for the severe temperature, pressureand reactant feed stream velocity conditions encountered in, forexample, many cracking processes.

Useful matrix materials include both synthetic and naturally occurringsubstances, as well as inorganic materials such as clay, silica and/ormetal oxides. The latter may be either naturally occurring or in theform of gelatinous precipitates or gels including mixtures of silica andmetal oxides. Naturally occurring clays which can be composited with thezeolite include those of the montmorillonite and kaolin families, whichfamilies include the sub-bentonites and the kaolins commonly known asDixie, McNamee-Georgia and Florida clays or others in which the mainmineral constituent is halloysite, kaolinite, dickite, nacrite oranauxite. Such clays can be used in the raw state as originally mined orinitially subjected to calcination, acid treatment or chemicalmodification.

In addition to the foregoing materials, the zeolites employed herein maybe composited with a porous matrix material, such as alumina,silica-alumina, silica-magnesia, silica-zirconia, silica-thoria,silica-beryllia, and silica-titania, as well as ternary compositions,such as silica-alumina-thoria, silica-alumina-zirconia,silica-alumina-magnesia and silica-magnesia-zirconia. The matrix may bein the form of a cogel. The relative proportions of zeolite componentand inorganic oxide gel matrix, on an anhydrous basis, may vary widelywith the zeolite content ranging from between about 1 to about 99percent by weight and more usually in the range of about 5 to about 80percent by weight of the dry composite.

When the catalyst compositions of the type hereinbefore described are tobe used for the conversion of aromatic compounds, the para-selectiveproperties of such catalysts can preferably be enhanced in known mannerby the treatment of such catalysts with oxides of a number of elementsprior to treatment with carbon dioxide in accordance with the presentinvention. Most commonly such catalysts are treated with phosphorusand/or magnesium compounds in the manner described in U.S. Pat. Nos.3,894,104; 4,049,573; 4,086,287; and 4,128,592, the disclosures of whichare incorporated herein by reference.

Phosphorus, for example, can be incorporated into such zeolites in theform of phosphorus oxide in an amount of from about 0.25% to about 25%by weight of the catalyst composition. Such incorporation can be readilyeffected by contacting the zeolite composite with a solution of anappropriate phosphorus compound, followed by drying and calcining toconvert the phosphorus compound to its oxide form.

Representative phosphorus-containing compounds which may be used includederivatives of groups represented by PX₃, RPX₂, R₂ PX, R₃ P, X₃ PO,(XO)₃ PO, (XO)₃ P, R₃ P=O, R₃ P=S, RPO₂, RPS₂, RP(O)(OX)₂, RP(S)(SX)₂,R₂ P(O)OX, R₂ P(S)SX, RP(SX)₂, ROP(OX)₂, RSP(SX)₂, (RS)₂ PSP(SR)₂, and(RO)₂ POP(OR)₂, where R is an alkyl or aryl, such as a phenyl radicaland X is hydrogen, R, or halide. These compounds include primary, RPH₂,secondary, R₂ PH and tertiary, R₃ P, phosphines such as butyl phosphine;the tertiary phosphine oxides R₃ PO, such as tributylphosphine oxide,the tertiary phosphine sulfides, R₃ PS, the primary, RP(O)(OX)₂, andsecondary, R₂ P(O)OX, phosphonic acids such as benzene phosphonic acid;the corresponding sulfur derivatives such as RP(S)(SX)₂ and R₂ P(S)SX,the esters of the phosphonic acids such as diethyl phosphonate, (RO)₂P(O)H, dialkyl alkyl phosphonates, (RO)₂ P(O)R, and alkyldialkylphosphinates, (RO)P(O)R₂ ; phosphinous acids, R₂ POX, such asdiethylphosphinous acid, primary, (RO)P(OX)₂, secondary, (RO)₂ POX, andtertiary, (RO)₃ P, phosphites; and esters thereof such as the monopropylester, alkyl dialkylphosphinites, (RO)PR₂, and dialkyl alkylphosphinite,(RO)₂ PR esters. Corresponding sulfur derivatives may also be employedincluding (RS)₂ P(S)H, (RS)₂ P(S)R, (RS)P(S)R₂, R₂ PSX, (RS)P(SX)₂,(RS)₂ PSX, (RS)₃ P, (RS)PR₂ and (RS)₂ PR. Examples of phosphite estersinclude trimethylphosphite, triethylphosphite, diisopropylphosphite,butylphosphite; and pyrophosphites such as tetraethylpyrophosphite. Thealkyl groups in the mentioned compounds contain from one to four carbonatoms.

Other suitable phosphorus-containing compounds include the phosphorushalides such as phosphorus trichloride, bromide, and iodide, alkylphosphorodichloridites, (RO)PCl₂, dialkyl phosphorochloridites, (RO)₂PCl, dialkylphosphinochloridites, R₂ PCl, alkylalkylphosphonochloridates, (RO)(R)P(O)Cl, dialkyl phosphinochloridates,R₂ P(O)Cl and RP(O)Cl₂. Applicable corresponding sulfur derivativesinclude (RS)PCl₂, (RS)₂ PCl, (RS)(R)P(S)Cl and R₂ P(S)Cl.

Preferred phosphorus-containing compounds include diphenyl phosphinechloride, trimethylphosphite and phosphorus trichloride, phosphoricacid, phenyl phosphine oxychloride, trimethylphosphate, diphenylphosphinous acid, diphenyl phosphinic acid, diethylchlorothiophosphate,methyl acid phosphate and other alcohol-P₂ O₅ reaction products.

Particularly preferred are ammonium phosphates, including ammoniumhydrogen phosphate, (NH₄)₂ HPO₄, and ammonium dihydrogen phosphate, NH₄H₂ PO₄.

Reaction of the zeolite with the phosphorus compound is effected bycontacting the zeolite compound with such compound. Where the treatingphosphorus compound is a liquid, such compound can be in a solution in asolvent at the time contact with the zeolite is effected. Any solventrelatively inert with respect to the treating compound and the zeolitemay be employed. Suitable solvents include water and aliphatic, aromaticor alcoholic liquids. Where the phosphorus-containing compound is, forexample, trimethylphosphite or liquid phosphorus trichloride, ahydrocarbon solvent such as n-octane may be employed. Thephosphorus-containing compound may be used without a solvent, i.e., maybe used as a neat liquid. Where the phosphorus-containing compound is inthe gaseous phase, such as where gaseous phosphorus trichloride isemployed, the treating compound can be used by itself or can be used inadmixture with a gaseous diluent relatively inert to thephosphorus-containing compound and the zeolite such as air or nitrogenor with an organic solvent, such as octane or toluene.

Prior to reacting the zeolite with the phosphorus-containing compound,the zeolite may be dried. Drying can be effected in the presence of air.Elevated temperatures may be employed. However, the temperature shouldnot be such that the crystal structure of the zeolite is destroyed.

Heating of the phosphorus-containing catalyst subsequent to preparationis also preferred. The heating can be carried out in the presence ofoxygen, for example, air. Heating can be at a temperature of about 150°C. However, higher temperatures, i.e., up to about 500° C. arepreferred. Heating is generally carried out for 3-5 hours but may beextended to 24 hours or longer. While heating temperatures about about500° C. can be employed, they are not necessary.

The amount of phosphorus incorporated with the zeolite should be atleast about 0.25 percent by weight. However, it is preferred that theamount of phosphorus in the zeolite be at least about 2 percent byweight when the same is combined with a binder, e.g., 35 weight percentof alumina. The amount of phosphorus can be as high as about 25 percentby weight or more depending on the amount and type of binder present.Preferably, the amount of phosphorus added to the zeolite is betweenabout 0.7 and about 15 percent by weight.

The amount of phosphorus incorporated with the zeolite by reaction withelemental phosphorus or phosphorus-containing compound will depend uponseveral factors. One of these is the reaction time, i.e., the time thatthe zeolite and the phosphorus-containing source are maintained incontact with each other. With greater reaction time, all other factorsbeing equal, a greater amount of phosphorus is incorporated with thezeolite. Other factors upon which the amount of phosphorus incorporatedwith the zeolite is dependent include reaction temperature,concentration of the treating compound in the reaction mixture, thedegree to which the zeolite has been dried prior to reaction with thephosphorus-containing compound, the conditions of drying of the zeoliteafter reaction of the zeolite composite with the treating compound, andthe amount and type of binder incorporated with the zeolite composite.

As noted, magnesium is another material commonly incorporated onto thezeolite composites of the present invention to thereby enhance theirpara-selectivity. Magnesium generally is incorporated as magnesium oxideand can be utilized either as the sole modifying agent or in combinationwith oxides of phosphorus as described hereinbefore or with othermaterials as described hereinafter. Incorporation of magnesium oxide isalso effected by contacting the zeolite composite with a suitablecompound of magnesium. Representative magnesium-containing compoundsinclude magnesium acetate, magnesium nitrate, magnesium benzoate,magnesium propionate, magnesium 2-ethylhexanoate, magnesium carbonate,magnesium formate, magnesium oxylate, magnesium amide, magnesiumbromide, magnesium hydride, magnesium lactate, magnesium laurate,magnesium oleate, magnesium palmitate, magnesium silicylate, magnesiumstearate and magnesium sulfide.

Reaction of the zeolite composition with the treating magnesium compoundis effected by contacting the zeolite with such compound. Where thetreating compound is a liquid, such compound can be in solution in asolvent at the time contact with the zeolite is effected. Any solventrelatively inert with respect to the treating magnesium compound and thezeolite may be employed. Suitable solvents include water and aliphatic,aromatic or alcoholic liquid. The treating compound may also be usedwithout a solvent, i.e., may be used as a neat liquid. Where thetreating compound is in the gaseous phase, it can be used by itself orcan be used in admixture with a gaseous diluent relatively inert to thetreating compound and the zeolite such as helium or nitrogen or with anorganic solvent, such as octane or toluene.

Heating of the magnesium compound impregnated catalyst compositionsubsequent to preparation is preferred. The heating can be carried outin the same manner and to the same extent as describeed hereinbeforewith respect to incorporation of phosphorus. After heating in air atelevated temperatures, the oxide form of magnesium is present.

The amount of magnesium oxide incorporated in the zeolite should be atleast about 0.25 percent by weight. However, it is preferred that theamount of magnesium oxide in the zeolite be at least about 1 percent byweight, particularly when the same is combined with a binder, e.g., 35weight percent of alumina. The amount of magnesium oxide can be as highas about 25 percent by weight or more depending on the amount and typebinder present. Preferably, the amount of magnesium oxide added to thezeolite is between about 1 and about 15 percent by weight.

The amount of magnesium oxide incorporated with the zeolite by reactionwith the treating solution and subsequent calcination in air will dependon several factors. One of these is the reaction time, i.e., the timethat the zeolite and the magnesium-containing source are maintained incontact with each other. With greater reactions times, all other factorsbeing equal, a greater amount of magnesium oxide is incorporated withthe zeolite. Other factors upon which the amount of magnesium oxideincorporated with the zeolite is dependent include reaction temperature,concentration of the treating compound in the reaction mixture, thedegree to which the zeolite has been dried prior to reaction with thetreating compound, the conditions of drying of the zeolite afterreaction of the zeolite with the magnesium compound and the amount andtype of binder incorporated with the zeolite.

In addition to treatment of the zeolite composites with phosphorusand/or magnesium as hereinbefore described in detail, such zeolites mayalso be treated with a variety of other oxide materials to enhancepara-selectivity. Such oxide materials include oxides of boron (U.S.Pat. No. 4,067,920); antimony (U.S. Pat. No. 3,979,472); beryllium (U.S.Pat. No. 4,260,843); Group VIIA metals (U.S. Pat. No. 4,275,256);alkaline earth metals (U.S. Pat. No. 4,288,647); Group IB metals (U.S.Pat. No. 4,276,438); Group VIB metals (U.S. Pat. No. 4,278,827); GroupVIA metals (U.S. Pat. No. 4,259,537); Group IA elements (U.S. Pat. No.4,329,533); cadmium (U.S. Ser. No. 139,611, filed Apr. 11, 1980 and nowabandoned); iron and/or cobalt (U.S. Ser. No. 150,868, filed May 19,1980 and now abandoned); Group IIIB metals (U.S. Pat. No. 4,276,437);Group IVA metals (U.S. Pat. No. 4,302,620); Group VA metals (U.S. Pat.No. 4,302,621); and Group IIIA elements (U.S. Pat. No. 4,302,622).

Treatment of the zeolite catalysts with any of the foregoing oxidematerials to enhance para-selectivity will generally occur before suchcatalyst are treated with carbon dioxide in accordance with the presentinvention in order to provide even greater enhancement of thepara-selective properties of such catalysts. Additional catalystmodifying procedures which may also optionally be employed to enhancecatalyst para-selectivity include precoking and steaming, orcombinations thereof.

Steaming entails contact of the zeolite with an atmosphere containingfrom about 5 to about 100 percent steam at a temperature of from about250° C. to about 1000° C. for a period of between about 15 minutes andabout 100 hours and under pressures ranging from sub-atmospheric toseveral hundred atmospheres. Preferably, steam treatment is effected ata temperature of between about 400° C. and about 700° C. for a period ofbetween about 1 and about 24 hours.

Precoking of the catalyst serves to deposit a coating of between about 2and about 75, and preferably between about 15 and about 75, weightpercent of coke thereon to enhance catalyst selectivity. Precoking canbe accomplished by contacting the catalyst with a hydrocarbon charge,e.g., toluene, under high severity conditions or alternatively at areduced hydrogen to hydrocarbon relative concentration, i.e., 0 to 1mole ratio of hydrogen to hydrocarbon, for a sufficient time to depositthe desired amount of coke thereon.

The zeolite catalyst composites described above, whether or not modifiedby treatment with phosphorus and/or magnesium oxides or other oxidematerials, or modified by steaming or precoking techniques, are treatedin accordance with the present invention with carbon dioxide to provideeven further enhancement of the para-selective properties of thecatalyst.

The catalysts described are treated with carbon dioxide under conditionswhich serve to enhance para-selectivity of the catalysts so treated.Catalyst treating conditions will vary with the concentration of thecarbon dioxide treating agent employed. However, in general suchpara-selectivity enhancing catalyst treating conditions will includecontact of the catalyst with a medium, preferably anhydrous, containingthe CO₂ treating agent, at temperatures of from about 50° C. to 500° C.for a period of about 0.1 to 25 hours, followed by calcination of thetreated catalyst at temperatures of from about 300° C. to 600° C.

The medium containing the carbon dioxide treating agent will generallywill generally be gaseous and will contain from about 25% to 100% byvolume of the CO₂ treating agent. Catalyst treatment operations may takeplace within the aromatics conversion reactor itself or may take placein a separate catalyst treatment vessel.

The treating medium is preferably passed over the catalyst at the rateof from about 20 to 100 cc/minute/gram catalyst at a temperature of fromabout 50° C. to 500° C. for a period of from about 0.25 to 3 hours.

It has been surprisingly discovered that treatment of the particularzeolite catalyst composites of this invention with the carbon dioxidetreating agent in the manner herein described will provide catalystshaving enhanced para-selective properties when used to promote theconversion of aromatic compounds to dialkyl substituted benzenecompounds. Such enhancement occurs even with catalysts which are alreadyhighly para-selective by virtue of having been treated with, forexample, phosphorus and/or magnesium compounds. Alternatively, treatmentof the zeolite catalysts herein with the particular carbon dioxidetreating agents of the present invention can permit elimination of theneed for steaming and/or precoking procedures in order to reach givenlevels of para-selectivity, particularly after regeneration of suchcatalysts with air or other oxygen-containing gas.

The treated zeolite catalysts of the present invention areadvantageously used to promote conversion of aromatic compounds toprovide dialkyl-substituted benzene product mixtures which are highlyenriched in the para-dialkyl substituted benzene isomer. Aromaticcompound conversion of this type includes alkylation, transalkylationand disproportionation.

Alkylation of aromatic compounds in the presence of the above-describedcatalyst is effected by contact of the aromatic with an alkylatingagent. A particularly preferred embodiment involves the alkylation oftoluene wherein the alkylating agents employed comprise methanol orother well known methylating agents or ethylene. The reaction is carriedout at a temperature of between about 250° C. and about 750° C.,preferably between about 300° C. and 650° C. At higher temperatures, thezeolites of high silica/alumina ratio are preferred. For example, ZSM-5having a SiO₂ /Al₂ O₃ ratio of 30 and upwards is exceptionally stable athigh temperatures. The reaction generally takes place at atmosphericpressure, but pressures within the approximate range of 10⁵ N/m² to 10⁷N/m² (1-100 atmospheres) may be employed.

Some non-limiting examples of suitable alkylating agents would includeolefins such as, for example, ethylene, propylene, butene, decene anddodecene, as well as formaldehyde, alkyl halides and alcohols, the alkylportion thereof having from 1 to 16 carbon atoms. Numerous otheraliphatic compounds having at least one reactive alkyl radical may beutilized as alkylating agents.

Aromatic compounds which may be selectively alkylated as describedherein would include any alkylatable aromatic hydrocarbon such as, forexample, benzene, ethylbenzene, toluene, dimethylbenzene,diethylbenzene, methylethylbenzene, propylbenzene, isopropylbenzene,isopropylmethylbenzene, or substantially any mono- or di-substitutedbenzenes which are alkylatable in the 4-position of the aromatic ring.

The molar ratio of alkylating agent to aromatic compound is generallybetween about 0.05 and about 5. For instance, when methanol is employedas the methylating agent and toluene is the aromatic, a suitable molarratio of methanol to toluene has been found to be approximately 0.1 to1.0 mole of methanol per mole of toluene. When ethylene is employed asthe alkylating agent and toluene is the aromatic, a suitable molar ratioof ethylene to toluene is approximately 0.05 to 2.5 moles of ethyleneper mole of toluene.

Alkylation is suitably accomplished utilizing a feed weight hourly spacevelocity (WHSV) of between about 1 and about 1000, and preferablybetween about 1 and about 200. The reaction product, consistingpredominantly of the 1,4-dialkyl isomer, e.g. 1,4-dimethylbenzene,1-ethyl-4-methylbenzene, etc., or a mixture of the 1,4- and 1,3-isomerstogether with comparatively smaller amounts of 1,2-dialkylbenzeneisomer, may be separated by any suitable means. Such means may include,for example, passing the reaction product stream through a watercondenser and subsequently passing the organic phase through a column inwhich chromatographic separation of the aromatic isomers isaccomplished. Alkylation using the carbon dioxide treated catalysts ofthe present invention can provide product mixtures containing at least90% or even 95% or more by weight of the para-dialkylbenzene isomer.

When transalkylation is to be accomplished, transalkylating agents arealkyl or polyalkyl aromatic hydrocarbons wherein alkyl may be composedof from 1 to about 5 carbon atoms, such as, for example, toluene,xylene, trimethylbenzene, triethylbenzene, dimethylethylbenzene,ethylbenzene, diethylbenzene, ethyltoluene, and so forth.

Another process embodiment of this invention relates to the selectivedisproportionation of alkylated aromatic compounds to producedialkylbenzenes wherein the yield of 1,4-dialkyl isomer is in excess ofthe normal equilibrium concentration. In this context, it should benoted that disproportionation is a special case of transalkylation inwhich the alkylatable hydrocarbon and the transalkylating agent are thesame compound, for example when toluene serves as the donor and acceptorof a transferred methyl group to produce benzene and xylene.

The transalkylation and disproportionation reactions are carried out bycontacting the reactants with the above described modified zeolitecatalyst at a temperature of between about 250° C. and 750° C. at apressure of between atmospheric (10⁵ N/m²) and about 100 atmospheres(10⁷ N/m²). The reactant feed WHSV will normally fall within the rangeof about 0.1 to about 50. Preferred alkylated aromatic compoundssuitable for utilization in the disproportionation embodiment comprisetoluene, ethylbenzene, propylbenzene or substantially anymono-substituted alkylbenzene. These aromatic compounds are selectivelyconverted to, respectively, 1,4-dimethylbenzene, 1,4-diethylbenzene,1,4-dipropylbenzene, or other 1,4-dialkylbenzene, as appropriate, withbenzene being a primary side product in each instance. The product isrecovered from the reactor effluent by conventional means, such asdistillation, to remove the desired products of benzene anddialkylbenzene, and any unreacted aromatic component is recycled forfurther reaction.

The hydrocarbon conversion processes described herein may be carried outas a batch type, semi-continuous or continuous operation utilizing afixed or moving bed catalyst system. The catalyst after use in a movingbed reactor is conducted to a regeneration zone wherein coke is burnedfrom the catalyst in an oxygen-containing atmosphere, e.g. air, at anelevated temperature, after which the regenerated catalyst is recycledto the conversion zone for further contact with the charge stock. In afixed bed reactor, regeneration is carried out in a conventional mannerwhere an inert gas containing a small amount of oxygen (0.5-2%) is usedto burn the coke in a controlled manner so as to limit the temperatureto a maximum of around 500°-550° C.

The following examples will serve to illustrate certain specificembodiments of the hereindisclosed invention. These examples should not,however, be construed as limiting the scope of the invention, as thereare many variations which may be made thereon without departing from thespirit of the disclosed invention, as those of skill in the art willrecognize.

EXAMPLE I

A base catalyst composition was prepared for use in evaluating thecarbon dioxide catalyst treating agent employed in this invention incomparison with known catalyst treating methods. To prepare such a basecatalyst, HZSM-5 zeolite (60.5 grams) having a crystal size of about 2microns in the form of 1/16 inch diameter extrudate with a 35 weightpercent alumina binder was steamed at 600° C. for 1 hour. The steamedmaterial was then impregnated with a solution of 38.7 grams ofdiammonium acid phosphate in 100 ml. of water, dried and calcined at500° C. for about 2 hours in an open dish. The resulting product wascooled and impregnated with a solution of 195 grams of magnesium acetatetetrahydrate in 133 ml. of water, dried and calcined at 500° C. forabout 16 hours. The final catalyst contained 4.93 weight percentmagnesium, present as the oxide, and 3.48 weight percent phosphorus,present as the oxide.

EXAMPLE II

A procedure was established to evaluate various test catalysts,including the base catalyst of Example I, for their performance incatalyzing para-selective aromatic conversion reactions. In accordancewith such a procedure, 2.2 grams of the test catalyst, 14-24 mesh, iscentered in a quartz reactor. Low surface area quartz chips are used toposition the catalyst and fill void spaces. After calcination with airat 500° C. for one hour, the temperature is adjusted to 425° C. Tolueneis fed to the reactor at a rate of 8.8 cc/hr. with a WHSV of 3.5. Atemperature rise occurs, and temperature is immediately adjusted to 450°C. After 25 minutes on stream at 450° C., the liquid product iscollected for a period of 5 minutes, for analysis. A 2 cc gas sample isalso taken at this time for analysis at a position just after the watercondenser. The temperature is then increased rapidly and successively to500° C., 550° C. and 600° C. In a similar manner, liquid and gaseoussamples are taken for analysis at each temperature during the last fiveminutes of a 30-minute run. This series of tests is used to determineperformance for selective toluene disproportionation to produce p-xyleneand benzene.

The reactor is then purged with nitrogen (without regeneration) and thetemperature adjusted to 375° C. Toluene is fed at a rate of 19.8 cc/hr,WHSV of 7.8, then ethylene is added at 15.6 cc/min., WHSV of 0.5, andthe nitrogen purge is stopped. The temperature is rapidly adjusted to400° C. In a similar manner, gaseous and liquid samples are taken duringthe last five minutes of a 30-minute run. An additional test run is madeat 450° C. This series of tests is used to determine performance for thealklyation of toluene with ethylene to produce p-ethyltoluene.

Using these catalyst evaluation procedures, the base catalyst preparedas in Example I was tested for its performance in the toluenedisproportionation and alkylation reactions described. Results for tworuns are provided in Table I.

                  TABLE I                                                         ______________________________________                                        Para-Selectivity of Mg/P Treated ZSM-5 Base Catalyst                                           SELECTIVITY TO PARA-                                                          ISOMER (% by weight)                                         REACTION           Run 1      Run 2                                           ______________________________________                                        Toluene Disproportionation                                                    450° C.     69.3       68.2                                            500° C.     65.9       64.3                                            550° C.     63.4       62.2                                            600° C.     57.0       60.7                                            Conversion Range:  1.1-11.0   1.1-12.7                                        (% by wt.)                                                                    Toluene Alkylation w/Ethylene                                                 400° C.     93.7       93.0                                            450° C.     92.2       92.2                                            Conversion Range:  16.5-12.4  14.8-13.2                                       ______________________________________                                    

The Table I data illustrate that the Mg/P-treated base catalyst sampleexhibits good para-selectivity which, unlike conversion, decreases withincrease in temperature. In accordance with prior art procedures, such acatalyst can be coke selectivated to increase para-selectivity to levelsof about 97% for toluene alkylation.

EXAMPLE III

To illustrate the catalyst treatment process of the present invention,the base catalyst as prepared in Example I is further treated by contactwith carbon dioxide. Prior to conversion testing, anhydrous carbondioxide is passed over the catalyst in the reactor at 100° C. for 15minutes at the rate of about 50 cc/min/2.2 g catalyst. This is followedby a brief purge and then by calcination in air for 1 hour at 500° C. Asimilar treatment is given a second catalyst sample except that CO₂contact is at 500° C. and lasts for 30 minutes. Catalyst samples sotreated are then tested for their disproportionation and alkylationperformance in the manner described in Example II. Results of suchtesting are provided in Table II.

                  TABLE II                                                        ______________________________________                                        Para-Selectivity of Mg/P ZSM-5 Catalyst                                       Treated With Carbon Dioxide                                                   Treating Agent                                                                              CO.sub.2      CO.sub.2                                          Treating-Procedure                                                                          15 min/100° C.                                                                       30 min/500° C.                             Run No.       1             2                                                 ______________________________________                                        TOL. DISPROPORT.                                                                            SELECTIVITY TO PARA ISOMER,                                                   WT %                                                            450° C.                                                                              78.0          79.2                                              500° C.                                                                              78.6          79.6                                              550° C.                                                                              74.0          74.7                                              600° C.                                                                              73.5          74.7                                              Conv. %       0.8-12.1      0.8-11.9                                          TOL + C.sub.2 H.sub.4                                                         400° C.                                                                              95.8          96.2                                              450° C.                                                                              94.7          95.2                                              Conv. %       14.6-12.6     15.4-12.8                                         ______________________________________                                    

The Table II data illustrate the increase in para-selectivity providedby treatment of the base catalyst with CO₂. Furthermore, it is evidentthat the decrease in para-selectivity with increased temperature wasreduced significantly by CO₂ treatment.

What is claimed is:
 1. A process for treating zeolite based catalysts,said catalysts comprising a crystalline zeolite material characterizedby a silica to alumina ratio of at least 12 and a constraint index offrom about 1 to 12 and from about 0.25% to 25% by weight of a modifyingoxide selected from magnesium oxide, phosphorus oxide and mixtures ofsaid oxides, said process comprising contacting said catalyst with acarbon dioxide treating agent under conditions capable of enhancing thepara-selectivity of said catalysts in the conversion of aromaticcompounds to dialkyl-substituted benzene compounds.
 2. A processaccording to claim 1 wherein said para-selectivity enhancing conditionsinclude contact of zeolite catalyst with treating agent at a temperatureof from about 50° C. to 500° C. for a period of from about 0.1 to 25hours, followed by calcination of the treated catalyst at a temperatureof from about 300° C. to 600° C.
 3. A process according to claim 2,wherein said zeolite is selected from ZSM-5, ZSM-11, ZSM-12, ZSM-23,ZSM-35, ZSM-38 and ZSM-48.
 4. A process according to claim 2 whereinsaid carbon dioxide treating agent is provided in a treating mediumcomprising from about 25% to 100% by volume of said treating agent.
 5. Aprocess according to claim 4 wherein said treating medium is passed oversaid catalyst at the rate of from about 20 to 100 cc/min/gm catalyst ata temperature of from about 50° C. to 500° C. for a period of from about0.25 to 3.0 hours.
 6. A process according to claim 5 wherein saidtreating medium comprises 100% by volume of anhydrous CO₂.
 7. A processaccording to claim 6 wherein said zeolite is ZSM-5.
 8. A catalystcomposition prepared in accordance with the process of claim 1, 2, 3, 4,5, 6 or
 7. 9. A catalyst composition according to claim 8 comprisingfrom about 1 to 99% by weight of zeolite material with the balance ofsaid composition comprising a binder for said zeolite material.